1. Field of the Invention
The present invention relates to a calibration method executed in an image forming apparatus.
2. Description of the Related Art
In general, an electrophotographic image forming apparatus requires calibration for adjusting the characteristics of an image to be formed into desired characteristics (Japanese Patent Laid-Open No. 2000-238341 (corresponding to U.S. Pat. No. 6,418,281)). Japanese Patent Laid-Open No. 2000-238341 describes a mechanism of creating a lookup table (LUT) used to perform density correction and gradation correction by reading an image pattern formed on a recording medium. Next, a reference density value is determined by measuring the density of a toner image formed on a photosensitive drum in accordance with the LUT. Lastly, the LUT is corrected by comparing the density value of a toner image formed again on the photosensitive drum at a predetermined timing with the reference density value. This makes it possible to maintain desired image density characteristics over a long period of time.
Japanese Patent Laid-Open No. 2000-238341 also brings about an effect of reducing the user's trouble and the number of recording media used, by executing a process of correcting the LUT more frequently than a process of creating an LUT using a recording medium. Hence, the invention disclosed in this patent reference is very excellent.
In recent years, the market demands that an image forming apparatus not only should achieve a faster operation and a performance for conserving more energy but also should cope with a variety of recording media from one with a small grammage to one with a large grammage. To cope with a wide range of grammages with limited power, the image forming speed (to be referred to as the process speed hereinafter) need only be changed for each type of recording medium. More specifically, a recording medium with a larger grammage need only be processed at a lower speed.
On the other hand, with a rising process speed, the difference between a maximum process speed and a minimum process speed is increasing. For example, the difference between a constant speed of 150 mm/s and its half speed is as low as 75 mm/s, but that between a constant speed of 300 mm/s and its half speed is as high as 150 mm/s. The difference in process speed varies by, for example, the dark decaying of the photosensitive body, the development efficiency, and the transfer efficiency, resulting in generation of a difference in gradation between different process speeds. It has been found that with such an increased speed difference, the use of a common LUT among a plurality of different process speeds generates a considerable difference between images formed at these process speeds. Under the circumstance, it is possible to adopt the invention described in Japanese Patent Laid-Open No. 2000-238341. Unfortunately, in this case, the user's trouble and the process time increase in proportion to the number of process speeds.